Laser alignment systems have been used for years, and some of these systems use a rotating laser light source that periodically sweeps 360°, thereby creating a plane of laser light that can be received and detected by a laser light receiver. Other types of laser alignment systems use a transmitter that projects laser energy continuously in a reference cone, or a reference plane, of light that spreads in all directions about a 360° circle. The range of such detection units, especially when used in direct sunlight, is greatly increased by modulating the laser light source at a predetermined frequency. Such a laser light alignment system using a modulated laser light source is disclosed in U.S. Pat. No. 4,756,617.
In this '617 patent, the laser energy in the alignment field is modulated at 8 kHz, and the signals produced by photodetectors of a laser light receiver are filtered and amplified. A split-cell photodetector is used, thereby producing two input signals that each have a particular strength, depending upon the position of the split-cell photodetector as it is being impacted by the modulated laser energy. After these two input signals are run through individual band pass filters and amplifiers having automatic gain control, the ratio of their signal strengths are compared (by a “ratio comparator” circuit 111) to determine the relative position that the laser light is impacting the photodetector receiver.
The synchronization aspect of the circuit in the '617 patent uses a phase locked loop to generate an internal oscillator signal that exactly matches the phase and frequency of the modulated received signal. The phase locked loop circuitry generates a control signal 136 (see FIG. 4) that feeds a pulse generator 150 (also on FIG. 4). Pulse generator 150 has some built-in time delay, and it outputs a pulse 107 that will occur approximately in the middle of the peak of the “pulse” or pseudo sine/square wave signal that is being received at the photodetectors. (By the time the received input signal comes through the band pass filter with gain, the square wave has been essentially converted into a pseudo sine wave.)
The '617 patent uses an “auto-correlator circuit” as its level detection means for the two channels of signals coming out of the two photodetectors. The auto-correlator uses an analog switch 106a for channel 1, and an analog switch 106b for channel 2. These two analog switches 106a and 106b are controlled by the output signal 107 of the pulse generator 150. When these analog switches 106a and 106b are turned ON (in their conductive state), they will pass the incoming signal from the band pass filter to two resistor-capacitor (RC) filter circuits, 108a and 108b, respectively. The incoming voltage from the band pass filter will charge the capacitors of these RC circuits, and cause the capacitors to be charged to the “peak” voltage of the pseudo sine waves. Then the analog switches are turned OFF, while the incoming waveform is still near its peak voltage, so that the voltage will remain on the capacitors for a certain time interval after the analog switches have been turned OFF. This essentially acts as a synchronous sample and hold circuit.
This auto-correlator approach of the '617 patent exhibits certain problems. One inherent problem is noise on the incoming sine wave. When the voltage waveform is sampled at its peak, any noise (e.g., from ambient light reflections) that occurs at that moment will tend to falsely charge the capacitor over its intended “peak” voltage for that input cycle. Therefore, the capacitor voltage will be incorrect, and this will result in elevation errors, and/or unstable elevation indications. Another inherent problem of this particular circuit design is that the sampled voltage that is placed on the capacitor will discharge over time into the ratio comparator 111. This is clear from the waveform 110 on FIG. 6. Therefore, the actual peak voltage will not be exhibited at the capacitor at the moment the ratio comparator samples the two incoming voltages stored on their respective capacitors on the downstream side of the analog switches 106a and 106b. 
One implementation problem with the auto-correlator circuit of the '617 patent is that the analog switches exhibit “charge injection,” which causes some spurious charge to flow toward the outputs of these analog switches when they are turned OFF. This thereby charges the “hold” capacitors of the RC filters on the downstream side of the analog switches 106a and 106b. This will, of course, tend to disturb the voltages on the capacitors, which therefore, may not be at the correct magnitudes. Another implementation problem of this auto-correlator circuit is when one of the two input channels has a bias voltage greater than the other. This will be perceived by the auto-correlator as a greater magnitude of the signal of interest, and actually occurs in devices manufactured according to this '617 patent.
It would be an improvement to provide a level detection current that overcomes the inherent problems and implementation problems that are exhibited in the design of the '617 patent, while at the same time being able to detect the relative magnitudes of modulated laser light signals that are received by the photocells of the receiver device.
It is known in the art to use synchronous demodulators or synchronous rectifier circuits in “lock-in amplifiers,” which typically are used to extract low amplitude signals that are modulated by a carrier signal in high-noise environments. Many of these lock-in amplifiers are used in laboratory settings, including with laser spectroscopy devices.